In recent years, along with the spread of PET bottle drinks, beverage manufacturers have been selling a large variety of products. These PET bottle products are decorated with design-focused labels for the purpose of clearly differentiating them from other brands, improving their image among customers, etc. For such labels, polystyrene-based shrinkable films are widely used. They are often wrapped around the body of the container during the production of PET bottle drinks. Recently, transparent and colorless labels (transparent and colorless labels refer to those on which only patterns are printed, rather than those on which full solid printing is performed) have also begun to be used from a design standpoint, e.g., harnessing the color of the content. Usually, the correct application of a label is checked using a label inspection machine. More specifically, the position of a printed label is confirmed by a CCD camera etc. However, when transparent and colorless labels are applied, it is difficult to confirm the label position, causing line troubles.    Patent Document 1: Japanese Unexamined Patent Publication No. 1996-80567